First try to restart the Plesk service to see if that clears the issue:

/etc/init.d/psa restart

If you see any of the following errors that means the Plesk password is out of sync with the MySQL database which holds that info:
*Unable to connect to database
*login.php3: Unable to connect to database: Permission denied
*ERROR 1045: Access denied for user: ‘admin@localhost’ (Using password: YES)

This does happen from time to time which is why Plesk provides a tool to re-sync this: